You can send a request to NJ Vital Statistics to get a copy of Zofia's death record. Besides giving you the exact date of death and the cause, it should give you where she was buried.
The fee is $25, plus $1 for each additional year searched. If you are positive she died by 1921, you can have them search in 1920 and 1921, which would cost you $26. If it's possible she died in 1922, you may want to pay the extra dollar and have them search that year as well (if they don't find her in 1920 or 1921, then you have to request a new search, which will cost another $25).
Since you are requesting a genealogical search, you do not have to provide proof of relationship--you just need to fill out as much info as you can, and provide a copy of your driver's license. Be patient though--it may take a really long time for them to send it to you.http://www.state.nj.us/health/vital/gen.shtml
This link will bring you to the page to get the form (Application form: (REG-28) [pdf 59k] [Word 58k]);http://www.state.nj.us/health/vital/nonexpeditedgen.shtml
Send applications via regular mail to:
New Jersey Bureau of Vital Statistics and Registration
Customer Service Unit – Genealogical Requests
PO Box 370
Trenton, NJ 08625-0370
If you want an expedited turnaround, you can do that through VitalChek, for an additional fee of course.